Theo Walcott has been told that he needs an abdominal operation which will rule him out for three weeks, to hinder both Arsenal and England. The winger is unsure how the injury took root but having carried it for a short while, he felt it more acutely in the warm-up before Sunday's home win over Stoke City and was immediately stood down.

He will miss a congested period for his club, beginning with Capital One Cup tie at West Bromwich Albion and also taking in the league fixtures at Swansea City and West Bromwich, which come either side of the Champions League game at home to Napoli. Worryingly for England, he will miss the World Cup qualifiers against Montenegro and Poland at Wembley on 11 and 15 October respectively.

Arsenal have described the procedure, which Walcott will undergo in Germany on Wednesday, as small and routine but there is the acknowledgement that the issue had to be addressed. The pain has stemmed from wear and tear – Walcott started all seven of Arsenal's previous matches this season – and it is a frustration to him and the manager, Arsène Wenger, who must contend with a lengthy injury-list.

He is already without Santi Cazorla, Tomas Rosicky, Lukas Podolski, Alex Oxlade-Chamberlain, Abou Diaby, Yaya Sanogo and Gedion Zelalem. Mikel Arteta, who came on as a late substitute against Stoke for his first football of the season after thigh trouble, is set to start at West Bromwich and Wenger is expected to play Nicklas Bendtner up front. There are also likely to be chances for Serge Gnabry, Ryo Miyaichi and Thomas Eisfeld.

Bendtner's return will be the tie's eye-catching feature. The Denmark striker, who has one year to run on his contract, had looked likely to depart in the summer only for proposed moves to break down. His wage demands proved to be a problem. Bendtner, though, said that he reached agreement with another club, only for Arsenal to block the transfer after they failed to borrow Demba Ba from Chelsea.

Wenger has spoken to the Dane and he has since been impressed with his application in training. With Olivier Giroud set to be rested, Bendtner appears to have edged in front of Chuba Akpom for the starting place. "He had nearly gone somewhere else and, in the end, he stayed because it didn't work out and we needed him here," Wenger said. "Now he can convince everybody. We are open-minded. It's just down to attitude and fighting spirit. When he has that, Nicklas can be an unstoppable striker. He gets his chance. Our job is about that. Take your chance."